It is well known to those skilled in the art that gelled or crosslinked water-soluble polymers are useful in enhanced oil recovery and other oil field operations. They have been used to alter the permeability of underground formations in order to enhance the effectiveness of water flooding operations. Generally, polymers along with an appropriate crosslinking system are injected in an aqueous solution into the formation. The polymers then permeate into and gel in the regions having the highest water permeability.
Because of environmental concerns as well as cost for disposing of a produced brine which is defined as the brine co-produced with oil and gas, it is desirable to utilize the produced brine as the aqueous solution used for the polymers and appropriate crosslinking system described above. Use of produced brines eliminates not only the cost associated with acquiring and pre-treating fresh water for use as the aqueous solution but, also the disposal cost for the produced brine. Most produced brines are known to be hard brines, i.e., those having a salinity of greater than 2% total dissolved solids, basically inorganic salts. Chromium(III) carboxylates such as, for example, chromium acetate are the only known crosslinkers which can be used to produce stable gels in produced brines for near-wellbore treatment. See for example R. D. Sydansk, Acrylamide-Polymer/Chromium(III) -Carboxylate Gels for Near Wellbore Matrix Treatments, Proceedings SPE/DOE Seventh Symposium on Enhanced Oil Recovery (1990). Although a chromium(III) salt is not as toxic as a chromium(VI) salt, it is not an environmentally desirable compound and its use may require additional costs to assure the integrity of the injection wells to avoid contamination of ground water sources.
There is, therefore, a need to develop a more environmentally suitable gelling composition that can form stable gels in produced brines for near-wellbore as well as in-depth treatments. It would also be a significant contribution to the art if a gelling composition does not crosslink too fast (i.e., gel times longer than 2 hours) so that individual components of the gelling composition can be injected simultaneously into the wells.